The present invention relates to a receptor layer transfer sheet and a thermal transfer sheet, and more specifically to a thermal transfer sheet and a receptor layer transfer sheet capable of providing images of high quality and high image density even on a transfer receiving material having an unsmooth surface.
The present invention also relates to a thermal transfer method and a thermal transfer apparatus, and more specifically to a thermal transfer method and an apparatus to be used therefor which are capable of providing images of high quality by using a thermal transfer system.
According to the present invention, it is possible to form gradation images such as photo-graphic images together with words and marks on ready made transfer receiving materials such as name cards, post cards, leaflets, curriculum vitaes, resumes, identification cards, licenses, commuter passes, membership cards, passports, notebooks, and coupon tickets.
Heretofore, various thermal transfer methods are known. Among these, there has been proposed a method wherein a sublimable dye (or subliming dye) is used as a recording agent, and is carried on a substrate sheet such as paper and plastic film to obtain a thermal transfer sheet, and various full color images are formed on a transfer receiving material such as paper and plastic film having thereon a dye receptor layer by using the resultant thermal transfer sheet. In such a case, a thermal head of a printer is used as heating means so that a large number of color dots of three or four colors are transferred to the transfer receiving material under heating in a very short period of time. As a result, a full color image of an original is reproduced by using the multi-color color dots.
The thus formed images are very clear and are excellent in transparency since the dyes are used therein as a colorant. Accordingly, these images are excellent in half tone reproducibility and gradation characteristic and are substantially the same as the images formed by the conventional offset printing and gravure printing. Further, when the above image forming method is used, there can be formed images of high quality which are comparable to full color photographic images.
In the above image forming method, however, the transfer receiving material on which the above mentioned images can be formed is restricted to a plastic sheet having a dyeing property (or dyeability) which is dyeable by a dye, paper on which a dye receptor layer has been formed in advance, etc.. Accordingly, the above mentioned method cannot provide an image directly on ordinary plain paper, etc... As a matter of course, when a receptor layer is formed on the surface of ordinary plain paper, the resultant paper can be subjected to such image formation. However, such a method generally requires a high cost, and it is difficult to apply this method to generally ready made transfer receiving materials such as post cards, memo papers, letter papers, and writing pads.
As a measure for solving such a problem, there is known a receptor layer transfer sheet which is capable of easily providing a dye receptor layer on an essential part (i.e., a part on which an image is to be formed) of the ready made transfer receiving material such as paper when an image is intended to be formed on the ready made transfer receiving material. As such a receptor layer transfer sheet, there has been proposed one comprising a substrate sheet having a releasability and a resin layer disposed thereon for forming a receptor layer, e.g., as disclosed in Japanese Laid Open Patent Application (JP-A, KOKAI) No. 264994/1987.
In a case where the receptor transfer sheet as described above is used so as to transfer the receptor layer to the transfer receiving material, substantially no problem is posed when the transfer receiving material comprises a coated paper having a smooth surface. However, when the transfer receiving material comprises plain paper, a post card, and other paper having a rough texture, the surface of such paper is composed of exposed fibers and is poor in surface smoothness. Accordingly, the receptor layer cannot uniformly be transferred to the surface of such paper and therefore white dropout or transfer failure occurs in the image formed on the resultant receptor layer, whereby high quality images cannot be obtained.
Further, when the receptor layer is partially transferred to the transfer receiving material so as to provide a small pattern or a pattern having a complicated configuration by means of a thermal head, etc., the film of the receptor layer is not necessarily cut properly so that the transfer thereof is not necessarily effected accurately.
In order to solve these problems, it is conceivable that the receptor layer is caused to have a large thickness (e.g., about 20 to 30 .mu.m) so that the surface unevenness of the paper is filled with the receptor layer. In practice, however, when the thickness of the receptor layer is increased, there occur various problems such that the thermal efficiency at the time of the transfer is lowered, cutting of the film becomes poor, and the film thickness becomes uneven. As a result, it is practically difficult to transfer the receptor layer per se, and the above problems cannot be solved.
As a measure for further simplifying the above operation, there has been proposed a thermal transfer sheet such that dye layers of yellow, magenta, and cyan (and optionally black, as desired) are sequentially formed on the surface of a continuous substrate film, and then a transfer receptor layer is formed on the same surface of the substrate film (Japanese Laid Open Patent Application Nos. 84281/1986 and 297184/1987). When such a thermal transfer sheet is used, the receptor layer is first transferred to a transfer receiving material, and then the dye layer of the respective colors are transferred to the receptor layer to form a full color image.
However, when the above thermal transfer sheet is used, it is required that the dye layer is firmly bonded to the substrate film, because the dye layer is liable to be transferred when the bonding therebetween is low. On the other hand, it is required that the receptor layer is bonded to the substrate film so as to provide an appropriate bonding strength. When the bonding strength is low, the peeling thereof is easy but the film cutting becomes poor. On the other hand, the bonding strength is too high, transfer failure occurs. As a result, the above requirements or performances for the dye layer and the receptor layer are antagonistic to each other.
There has also been proposed a method wherein a polyester film having a surface with an improved bonding property is used as a substrate film. However, the above antagonistic performances have not been satisfied even when such an improved polyester film is used.